(1) Field of the Invention
The present invention relates to a stabilized polyvinyl chloride resin composition. More particularly, the present invention relates to a non-toxic polyvinyl chloride resin composition comprising a hydrotalcite, magnesium oxide, a .beta.-diketone compound and an organic acid salt of zinc, or an organic tin compound, in which the trouble of bubbling at the molding step is eliminated.
(2) Description of the Prior Art
A polyvinyl chloride resin is a resin which is unstable against heat and light, and at the heat-molding step, thermal decomposition, which is due to dehydrochlorination, is readily caused, with the result that such disadvantages as discoloration of a molded product and reduction of the mechanical strength are brought about. In order to eliminate these disadvantages, it is necessary to add a heat stabilizer for controlling thermal decomposition, and organic compounds of metals such as Sn, Pb, Ca, Ba, Zn and Cd are used singly or in combination as the heat stabilizer. Moreover, various stabilizing assistants such as epoxy compounds, organic sulfur compounds, polyhydric alcohols and organic phosphorus compounds are used in combination with these stabilizers for improving the stabilizing effect.
With recent improvement of the living standard, the requirement for non-toxicity is increasing, and these stabilizers are reconsidered from the viewpoint of non-toxicity. When resins are used for food containers, food packaging films and medical appliances, non-toxicity of a high level is required for starting resins. As the non-toxic stabilizer meeting this requirement, there can be mentioned only organic tin compounds such as thioglycolic acid alkyl ester mercaptides of methyl tin, octyl tin and butoxycarbonyl tin, and composite metal soaps of the Ca/Zn system. The mercaptide type stabilizer of the organic tin compound, that can be used for foods, has a peculiar smell derived from mercaptan and injures the inherent flavor and quality of a food content. Since this problem of a bad smell is not involved in the Ca/Zn composite metal soap stabilizer, this stabilizer has been used in the field of packaging of drinking water, foods and alcohols. However, this stabilizer is defective in that a molded article is discolored mainly into yellow, the heat stability is poor and the transparency is poor.
Accordingly, development of a novel non-toxic smell-less stabilizer not causing discoloration in a molded article and having a high heat stability has been eagerly desired.
Under this background, we previously proposed a hydrotalcite as a novel heat stabilizer (Japanese Patent No. 1,213,856 and Japanese Patent Application Laid-Open Specification No. 80445/80). This hydrotalcite stabilizer is highly improved over the Ca/Zn composite metal soap stabilizer in heat stability and transparency, but the stabilizer is defective in that discoloration and bubbling are caused in a molded article. This defect of discoloration has been overcome by using a combination of a hydrotalcite as the stabilizer with an organic acid salt of zinc and a .beta.-diketone compound or its metal salt or with an organic tin compound (Japanese Patent Application Laid-Open Specification No. 80444/82). As means for overcoming the problem of bubbling, there has been tried a method using a product obtained by substantially removing water of crystallization from a hydrotalcite by heating it at a temperature not higher than about 300.degree. C. However, during the period of from the time of preparation of a stabilizer composition to the time of use of the stabilizer composition, absorption of water is caused and the problem is not practically solved. This problem of bubbling is especially serious in production of a rigid polyvinyl chloride resin molded article for which a molding temperature of about 200.degree. C. is adopted.